Methods for optical measurement of teeth and the subsequent automatic manufacture of dentures are generally known, e.g. from EP 0299490. Here the tooth is measured by means of an optical measuring method according to the Moiree, phase-shift or triangulation technique. In all these measuring methods it is assumed that the surface of the tooth reflects optimally, e.g. perfect diffusion (Lambert's reflection). Since this is not the case as a general rule, these methods regularly supply inaccurate data which result in the automatically manufactured dentures having bad fitting accuracy.
FIGS. 1a to 1c clearly show the different reflection qualities of a tooth and their effects with regard to determining the tooth coordinates. In FIG. 1a the surface of the tooth enamel is partly transparent (translucence) which results in the optical rays penetrating it. Thereby, in the measurement process, systematically too-small tooth dimensions are obtained. This effect depends on the direction of the ray striking the tooth surface relative to the tooth surface.
FIG. 1b shows a further optical difficulty when measuring the tooth, which is that the shiny tooth surface reflects partly directionally. This results in a displacement of the projected pattern and therefore also to inaccurate measurement of the tooth coordinates.
FIG. 1c, on the other hand, shows a perfectly-diffuse reflection as is required in the methods of the state of the art mentioned above.
These non-optimal reflection qualities of the tooth have been determined in the course of time, and thereby eliminated by coating the tooth to be measured by means of matting substances such as, e.g. chalk solutions, titanium dioxide powder or the like. Thus, for example, with the known CEREC method a measurement powder is sprayed onto the tooth to make its surface opaque and reflection-free. Furthermore disclosed in the state of the art (EP-A-0234422) is a method for improving the reflecting power. Therein the tooth is coated with a suspension that contains white pigments in an aqueous alcoholic solution in order in this way to improve the reflecting qualities of the tooth.
A disadvantage is that this pre-treatment increases the treatment time noticeably and is extremely unpleasant for the patient. Furthermore this method requires as thin and as uniform a layer thickness as is possible. Irregularities in the layer thickness are difficult to control and therefore lead to inaccuracies in the measurement data.